Can someone point me to some documentation on how to implement a Bittorrent UDP application?
I am working on a Bittorrent application and I can successfully download using TCP but I want to implement UDP and I can't find any information on it. I am retrieving peers with UDP trackers but that approach appears different than downloading files. If I sniff a UDP Handshake I see the following in Wireshark:

172.16.49.213 5.31.44.30 UDP 62 35507 → 18318 Len=20

5.31.44.30 172.16.49.213 UDP 62 18318 → 35507 Len=20

This is done before the handshake. Also, it looks like there are 20 bytes prepended to the Handshake (different than the 20 bytes above).

So I need some information to help me with what all this means.

1 Answer

The BitTorrent Peer Wire Protocol over UDP, called uTP is specified in:
BEP29 - uTorrent transport protocol

BitTorrentInc has also published uTP as a IETF RFC were they call it LEDBAT:
RFC 6817 - Low Extra Delay Background Transport (LEDBAT)

However, I wouldn't recommend anyone to do their own implementation, (except as a learning experience), as it involves a lot of time critical, low level network IO and is very tricky to get right.

Instead I recommend to use the library:
Almost all clients implementing uTP uses this library. AFAIK, the only exception is libtorrent/rasterbar (used by qBittorrent and Deluge) and it don't work as good as libutp does.

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